FR2998563A1 - PROCESS FOR THERAPY OR DEHYDRATION OF SLUDGE - Google Patents

Abstract

The subject of the invention is a process for thickening or dewatering a sludge comprising at least: a) a step of adding flocculants to the sludge to be treated; b) a step of flocculation by stirring the sludge thus added to form a mixture of flocs and an aqueous solution; c) a step of mechanical separation of the flocs and the aqueous solution formed in the previous step; d) a step of recovering the aqueous solution and flocs forming a treated sludge; characterized in that: the added flocculants consist of at least one cationic starch (A) and at least one cationic polyacrylamide (P); ? the cationic starch (s) (A) comprise a fixed nitrogen mass percentage of at least 2%; ? and the ratio of the masses (R) mA / (mA + mP) expressed in dry mass is between 0.6 and 0.99.

Description

Field of the Invention The present invention relates to sludge thickening or dewatering processes employing at least one cationic starch and one cationic polyacrylamide.

State of the art The treatment of waste water from human and industrial activities leads to the production of sludge. In general, the sludge to be treated consists mainly of water in which organic matter is intimately dispersed. This sludge undergoes an operation of mechanical separation of the organic matter and the water included in the sludge. These operations are included in thickening processes where the mechanical separation step is generally performed on a drip or flotation table or in sludge dewatering processes which often use centrifugation techniques, filter press or band filter. These processes are designed to obtain concentrated sludge of high dryness, that is to say, to concentrate the maximum dry matter of the sludge and to eliminate the water produced. The sludge thus treated is, depending on its origin, then transported to a landfill, agricultural spreading or incineration site. Current processes for thickening or dewatering the sludge require the addition of flocculants in the sludge to be treated. This will have the effect of producing concentrated organic matter in the form of flocs dispersed in water. These flocs are then separated from the water produced by the mechanical separation operations mentioned above. However, the latter can impose mechanical stresses on the flocs formed during their separation and it is therefore necessary that these flocs have the necessary mechanical strength to be able to undergo these constraints. Industrially, so far, mainly polyacrylamide polymers are used. Indeed, the concentrated sludge previously flocculated by these polymers generally have a very high dryness. In addition, the flocs have a very high resistance and thus the operation of thickening or mechanical dehydration is easily done. However, different problems are related to their use. First of all, these polymers are made from a monomer, acrylamide, which is included in Annex I of the classifications harmonized at European level by the 28th adaptation to technical progress (APT) of Directive 67/548 / EC of 6 August 2001, where it is notably classified as toxic if swallowed and may cause genetic alterations or cancer. The risks associated with the manufacture and use of acrylamide are therefore real. The presence of traces of monomer in the polymer also raises many controversies on the use of polyacrylamides, particularly in some countries such as Germany. In addition, the use of polyacrylamide for the treatment of sludge tends to clog the filters when they are used to separate the flocs from the water. Another problem is also that these polyacrylamides have a relatively high cost.

Another solution is to use, instead of polyacrylamides, cationic starches. These products have the advantage of being made from starch, which is a renewable resource. These starch-based flocculants make it possible to solve the problems of polyacrylamide insofar as they are less expensive and, generally speaking, are manufactured from products that are less toxic to humans or their environment. Another advantage is that, during a step of separation of flocks and water by filtration, it is not observed clogging filters when using these cationic starches as flocculating agent, unlike when using polyacrylamides. However, flocs formed using these cationic starches have very low strength compared to those obtained from polyacrylamide. The concentrated sludge obtained at the end of the thickening or dehydration stage also has the disadvantage of having a lower dryness than in the case where polyacrylamide is used. It is also already known to use two opposite charge flocculants in a sequence process. Thus, in US Pat. No. 4,382,864, a sludge dewatering process is described in which a second flocculation step is added by adding a first flocculant with vigorous stirring and a second flocculation step is added by adding a slurry. second flocculant of the opposite charge to the first under a lower agitation to increase the size of the flocs. Flocculants, anionic and cationic, may be a polyacrylamide and a starch. This method, comprising at least two successive flocculation stages, is relatively complex to set up and requires a precise adjustment of the stirring speed for each of the flocculation steps. SUMMARY OF THE INVENTION Thus, to date, there still exists a need to find thickening or dewatering processes, using small amounts of polyacrylamide and which can be easily implemented, these methods making it possible to obtain high sludge of dryness. and wherein the flocs formed in this process have sufficient mechanical strength to allow easy mechanical separation of the flocs and water. This is precisely what allows the present invention which relates to a method of thickening or dewatering a sludge comprising at least: a) a flocculant addition step in the sludge to be treated; b) a step of flocculation by stirring the sludge thus added to form a mixture of flocs and an aqueous solution; c) a step of mechanical separation of the flocs and the aqueous solution formed in the previous step; d) a step of recovering the aqueous solution and flocs forming a treated sludge; in which: the added flocculants consist of at least one cationic starch (A) and at least one cationic polyacrylamide (P); the cationic starch (s) (A) comprise a fixed nitrogen mass percentage of at least 2%; and the ratio of the masses (R) mA / (mA + mp) expressed in dry mass is between 0.6 and 0.99.

This process, using flocculants of the same cationic charge, makes it possible to obtain an excellent compromise and to approach, equalize or even exceed the results obtained with a process using only a cationic starch or a cationic polyacrylamide as flocculant.

DETAILED DESCRIPTION OF THE INVENTION The invention relates to a sludge treatment process, more specifically a thickening or dewatering process, in which different cationic flocculants of the sludge are used, that is to say, predominantly from the sludge. cationic starch (A) and in a minor amount cationic polyacrylamide (P). The cationic starch comprises a fixed nitrogen mass percentage of at least 2. The cationic starch used in the context of the invention can be obtained from any type of native starch of natural or hybrid origin, including starch derived from plant organisms having undergone genetic mutations or manipulations. Said starches may in particular be derived from potato, high amylopectin potato (waxy potato), wheat, high amylopectin wheat (waxy wheat), corn, high grade corn amylopectin (waxy corn), high amylose maize, rice, peas, barley or cassava, cuts or fractions thereof, and any mixtures of any two or more of the products above. The selection of this native starch has, for example, an influence on the final molecular weight as well as on its branching rate, related to the content of amylose and amylopectin.

According to the invention, the cationic starch is preferably obtained from corn starch, wheat, waxy corn or from potato starch, most preferably from potato starch. The cationization reaction can be carried out using cationic reagents as described for example in "Starch Chemistry and Technology" - Vol. He - Chapter XVI - R. L. WHISTLER and E. F. PASCHALL - Academic Press (1967). The starch is introduced into a reactor in the presence of these reagents. The reaction is generally carried out in the glue phase. By "glue phase" is meant that the starch is at least partially solubilized, generally completely solubilized, in a solvent phase, said solvent phase generally being an aqueous phase or a hydroalcoholic phase. At the end of this process, a cationic starch is obtained in the form of a liquid starchy composition. It is also possible to obtain the cationic starch in solid form by drying the composition or else by precipitation in alcohol or a hydroalcoholic solvent.

Preferably, the cationization reaction is carried out with nitrogen-containing reagents based on tertiary amines or quaternary ammonium salts. Among these reagents, it is preferred to use 2-dialkylaminochlorethane hydrochlorides such as 2-diethylaminochloroethane hydrochloride or glycidyltrimethylammonium halides and their halohydrins, such as N- (3-chloro-2-hydroxypropyl) -trimethylammonium chloride this latter reagent being preferred. This reaction is generally carried out in an alkaline medium, at a pH greater than 8, or even greater than 10, the pH being adjustable for example by sodium hydroxide. The levels of reagent employed are chosen such that the resulting cationic starches have the percentage of nitrogen that has been fixed on the starch. The cationic starch (A) according to the invention has a fixed nitrogen mass percentage of at least 2%. Preferably, the cationic starch (A) has a fixed nitrogen mass percentage ranging from 2.5 to 5%. The amounts of fixed nitrogen are conventionally determined by the Kjeldahl method. Without limitation, the methods described in the documents WO 2007121981, EP 737210, WO 9962957, US 2995513, US 3842005 and US 4579944 may be used. According to the invention, the cationic starch may be added during the step a) the process via a liquid starchy composition comprising at least one cationic starch. This variant has the advantage of being easy to implement in industrial installations. Preferably, the mass concentration of cationic starch of this composition added during step a) ranges from 0.1 to 20 g / l. To make this starchy composition, it may be necessary to solubilize the cationic starch in the solvent if this cationic starch is in solid form. The cationic starch (A) useful in the invention may be soluble in cold water. It may for example be in the form of a powder soluble in cold water. The liquid starchy composition is generally an aqueous composition, which may comprise mainly as a solvent water and possibly small amounts of organic solvents miscible with water, such as alcohols such as ethanol, for example in amounts of solvent less than 10% by weight of all the solvents. However, it is also possible to use a liquid starchy composition derived from the glue phase cationization process previously described. According to an advantageous variant of the invention, a cationic starch-free liquid cationic starch composition containing a preservative is used.

When the cationic starch is in the form of a liquid starchy composition, microbial degradation can be observed during its storage and transportation of the product. To limit this phenomenon, it is possible to add a biocidal agent, which may be chosen from phthalates, for example one of those marketed by the Dow Chemical Company under the trademark VINYZENETM. However, although the concentration of biocidal agent necessary for the storage of the starch in the form of a liquid solution is low, these biocidal agents can constitute unwanted constituents for the treatment of a sludge, the treated sludge being able, for example, to be spread at the end of the treatment process. The fact that the starch is stored and transported in solid form limits the problems of degradation. This makes it possible to dispense with the addition of a preservative, which may be particularly advantageous in a water treatment process. The starch composition comprising the cationic starch may further comprise additional components, such as biocidal agents described above. According to the process of the invention, the polyacrylamide (P) used is cationic polyacrylamide. As regards the cationic polyacrylamide, it is preferably of high molecular weight, in particular of molecular weight Mn of at least 1.106 g.mol -1. Advantageously, the molecular weight of the polyacrylamide Mn ranges from 1.106 to 20.106 g.mol -1, in particular from 1.106 to 15.106 g.mol -1 -1. The cationic polyacrylamide used may be in a linear form or in a branched form. Said polyacrylamide may be cationic up to 100% filler and, preferably, between 0.1 and 55%, for example between 0.1 and 25%, filler. Alternatively, the degree of cationicity of the polyacrylamide can be quantified using a Streaming Current Detector (SCD) type flow detector, by titration with sodium polyethylenesulphonate. Preferentially, the cationicity level of the polyacrylamide ranges from 500 to 5000 peq / g, preferably from 2000 to 4500 peq / g.

The cationic polyacrylamide may be chosen from polyacrylamide copolymers with cationic monomers or polyacrylamides modified according to the Mannich reaction. Examples of cationic polyacrylamide copolymers that may be mentioned include acrylamide / diallyldialkylammonium halide copolymers, preferably diallyldialkylammonium chloride, diaminoalkyl (meth) acrylate / acrylamide copolymers and dialkylaminoalkyl (meth) acrylate / acrylamide copolymers, trialkylaminoalkyl (meth) copolymers acrylate, the alkyl groups preferably comprising from 1 to 6 carbon atoms. According to the invention, the cationic polyacrylamide may be added during step a) of the process via a liquid composition comprising at least one cationic polyacrylamide. This variant has the advantage of being easy to implement in industrial installations. Preferably, the mass concentration of cationic polyacrylamide in this composition ranges from 0.1 to 20 g / l. The cationic polyacrylamide useful in the invention may be soluble in cold water. It may for example be in the form of a powder soluble in cold water.

The cationic polyacrylamide liquid composition is generally an aqueous composition, which may comprise mainly as a solvent water and possibly small amounts of water-miscible organic solvents, such as alcohols such as ethanol, for example in amounts of organic solvent less than 10% by weight of all the solvents.

According to the invention, the cationic starch and the cationic polyacrylamide are added in the sludge to be treated during step a) in a ratio of the masses (R) mA / (mA + mp), expressed in dry mass, between 0.6 and 0.99. Unless explicitly stated, it is stated that the quantities of cationic starch and polyacrylamide, and therefore of flocculants, are expressed in dry mass.

R is advantageously between 0.7 and 0.97, preferably between 0.85 and 0.95. In the process according to the invention, the cationic starch and the cationic polyacrylamide can be added separately or simultaneously during step a). According to the variant in which the flocculants are added simultaneously, the addition step can be carried out via the addition of a liquid composition comprising at least one cationic starch and at least one cationic polyacrylamide. Flocculants are used in the process of the invention in amounts to flocculate the sludge to be treated. These amounts may be adapted by the skilled person depending on the type of sludge to be treated. In the same way, the quantities will be adapted by those skilled in the art depending on the desired floc strength to be able to satisfactorily perform the flocculation steps b) and mechanical separation c) which are detailed below. The quantities of sludge to be treated with the flocculants are also expressed as the dry mass of sludge to be treated. According to the process of the invention, the total amount of cationic starch and of polyacrylamide can range from 1 to 50 g / kg of sludge to be treated (dry / dry), advantageously from 3 to 35 g / kg (dry / dry). preferably from 4 to 20 g / kg (dry / dry). According to the process of the invention, the flocculants can be added to the sludge in a single dose, or by the addition of different doses. In the case of a continuous sludge treatment process, it is possible to add the sludge to be treated by the continuous addition of flocculants. Although their use is not necessary to carry out the process according to the invention, it is also possible to add, during step a), other products conventionally used in the treatment of sludges, in particular coagulants, or even one or more flocculants. other than the cationic polyacrylamide (P) and cationic starch (A) useful for the invention. These other flocculants, defined according to the present application as "additional flocculants" may be anionic or cationic polysaccharides other than cationic starches (A) or may be anionic polyacrylamides.

Advantageously, the mass quantities of additional flocculants are less than 50 g / kg (dry / dry) of sludge to be treated, advantageously less than 30 g / kg (dry / dry), preferably less than 10 g / kg (dry / dry). . Most preferably, no additional flocculants are added in the process of the invention.

The sludge to be treated in step a) can be of any type and in particular be derived from any type of human and industrial activity. The method according to the invention is particularly useful when the sludge comes from urban effluents or when it comes to industrial sludge, especially sludge from the food industry such as sludge from starch mills, paper mills, of chemical industries. The process according to the invention is particularly effective for sludges comprising organic matter in suspension.

According to the process of the invention, the sludge to be treated can, before addition of the flocculants in step a), undergo a pretreatment, for example a pretreatment of digestion by bacteria or pretreatment with chemical agents. In particular, the sludge to be treated may, according to the process of the invention, be pretreated with coagulating agents, for example with the aid of metal salts. The sludge to be treated is generally an aqueous composition comprising water and suspended solids, the amount of suspended solids being greater than or equal to 0.2% of the mass of the aqueous composition. Step b) of the process consists of a step of flocculation by stirring the sludge added by the flocculants to form a mixture of flocs and an aqueous solution.

This step is conventionally conducted in a flocculator. As previously explained, cationic starch (A) and polyacrylamide (P) can be introduced separately, with a delay between each of the introductions. This time is generally less than a few minutes, for example less than 1 minute. It is therefore possible to carry out a first flocculation step after a first addition step of a first flocculant and then a second flocculation step after a second addition step of a second flocculant. However, it is preferable, for simplicity of the process, to conduct only one flocculation step b) after a step of adding a) a composition comprising (A) and (P).

The subject of the invention is therefore also a composition which can be used in the process according to the invention, comprising at least one cationic starch (A) and at least one cationic polyacrylamide (P) in which: cationic starches (A) comprise a fixed nitrogen mass percentage of at least 2 (3/0); and the mass ratio (R) mA / (mA + mp) expressed in dry mass, is between 0.6 and The preferred variants described above for the process according to the invention concerning (A), (P) and their relative proportions are also preferred variants of the composition according to the invention The composition according to the invention is generally under form of a liquid composition comprising at least one cationic starch and at least one cationic polyacrylamide solubilized.This composition according to the invention has the advantage of being easy to implement in industrial facilities. According to the invention, a cationic starch in solid form can be solubilized in a solvent. The cationic starch useful in the invention may be soluble in cold water. It may for example be in the form of a powder soluble in cold water. The liquid composition according to the invention can solubilize a cationic starch and a cationic polyacrylamide, both in solid form, in a solvent. The liquid composition according to the invention can also be manufactured by solubilizing a cationic polyacrylamide in a cationic starch adhesive. The liquid composition according to the invention is generally an aqueous composition, which may comprise mainly as solvent water and possibly small amounts of water-miscible organic solvents, such as alcohols such as ethanol, for example in amounts of organic solvent less than 10% by weight of all the solvents.

It is preferred that, relative to the total mass of the composition, the mass amount of the sum of cationic starch and cationic polyacrylamide ranges from 0.01 to 70%, advantageously from 20 to 60% and preferably from 30 to 50%. . When it is desired to transport or store this composition, it is preferred that the mass quantity of the sum of cationic starch and cationic polyacrylamide in said composition ranges from 10 to 70%, advantageously from 20 to 60%, preferably from 30 to 50%. %. When it is desired to use the composition in the process according to the invention, it is preferred that the mass amount of the sum of cationic starch and cationic polyacrylamide ranges from 0.01 to 2%.

According to the process of the invention, the flocculated sludge obtained in step b) is subjected to at least one step c) mechanical separation, in order to separate flocs and aqueous solution obtained previously. According to one embodiment, this step c) is carried out by treating the flocculated sludge obtained in step b) by passing it over a drip, centrifugation and / or flotation table. These techniques are generally used when it is desired to carry out a method of thickening the sludge. These processes generally make it possible to obtain an average dryness of the sludge, this dryness ranging from 4 to 10%, often from 5 to 7%. The sludge thus obtained is generally in liquid form.

According to another embodiment, this step c) is carried out by treating the flocculated sludge obtained in step b) by centrifugation and / or by passing through a filter press or band filter. These techniques are generally used when it is desired to carry out a method of dewatering the sludge. These methods allow, according to the technique used, to obtain a high dryness of the sludge, this dryness being generally greater than 10% and up to 40%. It should be noted that centrifugation is a technique which, depending on the setting of the centrifuge chosen from those skilled in the art, to obtain a dryness of the medium or high treated sludge.

According to the invention, the method may comprise different successive separation steps. By way of example, it is possible to carry out a sludge dewatering process comprising a first separation step by passing through a drip, centrifugation and / or flotation table and then a second separation stage by centrifugation and / or by passage on filter press or band filter. Thanks to the particular combination of flocculants of polyacrylamide type and starch, the method according to the invention makes it possible to remove large quantities of water in the sludge to be treated, with results approaching or exceeding for certain properties, those obtained with similar processes using exclusively cationic polyacrylamides. This thus makes it possible to use smaller amounts of polyacrylamide compared to the methods already known while retaining an efficient process. In particular, the sludge treated according to the process of the invention may present, after step d), a dryness of at least 4%, generally ranging from 4 to 60%, generally not exceeding 40%, advantageously greater than 10%, preferably at least 20%. According to the method of the invention, the successive steps a), b), c) and d) can be carried out one or more times. This means that the treated sludge recovered in step d) may possibly undergo another treatment of the successive steps a), b), c) and d).

The invention will now be illustrated in the following nonlimiting examples of the present invention. EXAMPLE 1 Study of the Effect of the Cationicity of Starch Solutions of cationized potato starch are prepared by following the teaching of US Pat. No. 4,577,9944, these starches having different levels of cationicity, expressed as a percentage of nitrogen. (% N), this percentage being specified in Table 1. Table 1 Product% N Starch 1 1.2 Starch 2 2.0 Starch 3 3.0 Starch 4 4.0 Also used as flocculant cationic polyacrylamide (FLOPAM DW 2160 marketed by the company SNF, in powder form with 83.5% dry matter, having an ionic charge (SCD) of + 2900eq / g), hereinafter referred to as PAM. It is previously prepared as a 1% solution with magnetic stirring for 1 hour before use for the treatment of sludge. These cationic starch solutions are tested in Jar-Test as a flocculant for sludge from a starch plant, together with the cationic polyacrylamide, with a mass ratio R mA / (mA + mp) equal to 0, 8 (dry / dry), for a total of 18 grams of dry product per kilogram of dry sludge (g / kg of dry sludge). A reference test is performed with polyacrylamide alone at an equivalent dose, ie 3.6 g / kg of dry sludge. The characteristics of the sludge to be treated are given in Table 2. Table 2 Dry matter 1.5% Chemical oxygen demand 18.30 g / L Suspended matter 18.10 g / L Calcium 604 mg / kg The polyacrylamide solution is first added with stirring for 10 seconds at 200 revolutions per minute (rpm). Then, the starch solution is added with stirring for 30 seconds at 45 rpm. The flocculation results are then expressed in floc size in the beaker (score from 0 to 10: 0 = no visible flocs, 10 = very large flocs). Then, the flocculated sludge is filtered on a mesh screen 500 μm; the following are then evaluated: - the quality of the flocks retained on the sieve (score from 0 to 10: 0 = no visible flocs, 10 = very large flocs); the filtration rate through the sieve (in volume collected over time, in ml / s); the quantity of flocs passing through the sieve (`) / 0 mass of flocks in the filtrate); - the turbidity of the filtrate (in UTN).

The results are given in Table 3. Table 3% N Floc Floc Turbidity% Volume of filtrate (ml / s) Beaker Starch Sieve Floc filtrate (UTN) in the filtrate 10s 30s 300s Without 2 1 70.0 56.7 150 190 300 starch (PAM) 1.2 3 2 17.0 14.0 130 180 285 2.0 6 3 18.0 8.3 210 260 300 3.0 8 5 14.0 6.3 260 290 315 4, 0 9 8 16.0 3.2 270 290 310 A marked improvement in flocculation is observed when the nitrogen content of the starch is greater than or equal to 2%. In particular, the starch improves the flocculant capacity so as to generate more floc large enough to be retained on the sieve. The filtration is fast since 70% of the volume of filtrate has passed through the sieve after only 10 seconds. In addition, the percentage of floc in the filtrate is less than 10%, which is similar to the values obtained with synthetic flocculants on the market. Polyacrylamide does not explain only such efficiency since at equivalent dose in the absence of starch the result is poor. EXAMPLE 2 Study of a Simultaneous or Separate Addition In this example, flocculants based on starch 4 and PAM are tested in a test jar according to two modes of addition, at different ratios. The total dose of flocculants is 32 g / kg. The sludge is the same as in the previous example, as well as the protocol with the exception of time and stirring speed. Addition in 1 time ("Time 1"): Addition of the two products at the same time with stirring at 200 rpm for 5s, via a single composition comprising PAM and starch. - Addition in 2 times ("Time 2"): Adding the PAM to a stirring of 200 rpm for 10s then adding the starch stirring at 45 rpm for 40s. The percentage of efficiency is determined, with regard to the resistance of the flocks in rheology, but also the percentage of flocs in the filtrate.

The measurement of the resistance of the flocs consists in measuring the stress at which the flocks are destroyed when the flocks retained on the sieve are tested on an AR2000 rheometer using a measurement geometry of concentric cylinders type at a frequency of 1 Hz at 20 ° C, the stress increasing from 0.02 to 880 Pa.

In order to facilitate the interpretation of the results, the results are given in terms of percentage of effectiveness. This percentage efficiency is calculated with respect to the 100% reference value, which is the stress obtained for the floc resistance obtained by the method comprising the addition of flocculants, in a time, for the starch / PAM comprising 98% by weight of starch. This reference was chosen because it was observed that at this dose, sludge treatment is performed very easily and quickly, especially in terms of filtering flakes on the sieve. When this efficiency becomes too low, the flocks may have a tendency to disintegrate and / or the flocks may pass through the sieve and the treatment of the sludge becomes more difficult. It has however been observed that an efficiency of less than 100% could also be sufficient to treat the sludge satisfactorily. The calculated efficiency percentage makes it easy to classify the various treatments. The results are given in Table 4. TABLE 4 Cyo (3/0% efficiency floc in the starch filtrate in the mixture Time 1 Time 2 Time 1 Time 2 98 100 64 3.1 6.3 85 128 The results show the advantage of using a single flocculant composition for the treatment of sludge according to the invention since the results are improved when the products are added. simultaneously.

In general, it has also been found that a good resistance of the flocs makes it possible to obtain flocks that do not pass through the sieve. Good floc resistance is therefore a good indicator for comparing different sludge treatment processes with each other.

Example 3 Effect of the R Weight Ratio and Effect of the Flocculant Amount A treatment plant sludge is treated with each of the four cationic potato starch solutions of Example 1 or using a cationic polyacrylamide at different doses ranging from 4 to 32 g of flocculant per kg of dry sludge to be treated. The slurry and the protocol are the same as in Example 1.

As in Example 2, the percentage of effectiveness for the resistance of the flocs is measured. This constraint is expressed as a percentage relative to the reference value of 100% obtained for flocs obtained with PAM, at a dose of 16 g / kg. This reference was chosen because it was observed that at this dose, sludge treatment is performed very easily and quickly, especially in terms of filtering flakes on the sieve.

The results for PAM alone are given in Table 5. Table 5% Flocculant dose in g / kg of efficacy 4 8 16 32 PAM 4 32 100 126 The results for solutions of starches alone are given in the table 6. Table 6% Flocculant dose in g / kg of efficacy 4 8 16 24 32 STARCH 1 3 4 8 10 16 STARCH 2 3 6 16 20 26 STARCH 3 3 10 20 32 40 STARCH 4 2 5 20 25 3220 Can be seen here that, whatever the cationic starch and the dosage, one can not reach the efficiency of 100%. An identical sludge treatment process is now carried out with the difference that a mixture of cationic starch and PAM is used in place of the starch solution or PAM. The starch / PAM mass ratio and the dosage of this mixture are varied. The results for the mixtures containing PAM and the STARCH 4 solution are given in Table 7. Table 7 Cyo Dose Dose Total starch cyo in the PAM1 starch mixture of effectiveness (g / kg) ( g / kg) (g / kg) 8 95 7,6 0,4 16 8 80 6,4 1,6 25 8 70 5,6 2,4 40 8 50 4 4 32 16 95 15,2 0,8 50 16 90 14.4 1.6 60 16 80 12.8 3.2 80 16 70 11.2 4.8 100 16 50 8 8 200 32 95 30.4 1.6 160 32 90 28.8 3.2 200 32 80 25.6 6.4 25010 As explained previously, a 100% efficiency makes it possible to perform an excellent treatment of the sludge. However, this value is only a reference and it is quite possible to treat the sludge satisfactorily by obtaining a resistance of less than 100%.

From the above results, it is concluded that it is therefore possible to obtain an efficiency of 50% with a dosage of 16 g / kg, using a mixture of flocculants cationic starch / PAM comprising only 5% of PAM (0, 8 g of PAM per kg of treated sludge). This efficiency is for example greater than that obtained for a treatment in which the flocculant consists solely of PAM, at a dosage of 8 g / kg.

At an equivalent dosage of 16 g / kg, treatment with a flocculant mixture comprising 30% PAM (4.8 g PAM per kg treated sludge) is as effective as treatment with flocculant consisting of PAM alone. If 100% efficacy is desired, treatment with a minimum dose of MAP (1.6 g / kg) can also be achieved using a flocculant mixture having an R ratio of 0.95 at a dose of 32 g per kg of dry sludge. It is concluded that the highly effective sludge treatment can be carried out with the aid of the process according to the invention, using very limited quantities of polyacrylamide. EXAMPLE 5 Treatment of Paper Mud Starch 4 and PAM are tested in Jar-Test as a flocculant for the treatment of paper sludge. The characteristics of the sludge given in Table 8. Table 8 Dry matter 2.3% Chemical oxygen demand 14.50 g / L Suspended material 21.74 g / L Calcium 4700 mg / kg crude The solution of STARCH 4 and the PAM solution are tested alone or mixed, at a variable dose.

As in the previous example, the percentage of effectiveness is determined relative to the reference value of 100% obtained for the flocs obtained with the PAM, at a dose of 16 g / kg. For ease of comparison, a percentage theoretical efficiency is calculated for the processes using a mixture, this theoretical percentage being calculated by averaging the percentages of efficiency of the processes using either starch alone or PAM alone. The results are given in Table 9. Table 9 Products Dose 0/0 0/0 Flocculant Effectiveness Efficiency (g / kg) Theoretical Raw Mud 0 0% PAM 4 20% 16 100% 32 125% STARCH 4 4 8% 16 25% 32 15% WFP / STARCH 4 (10/90) 4 16% 9.2% 16 40% 32.5% 32 125% 26% On this sludge, the percentage of effectiveness of STARCH 4 used alone remains low, regardless of the amount of flocculant used. PAM makes it possible to provide an efficient sludge treatment method. In the case of processes using mixtures, it is possible to improve in all cases the resistance of the flocs with respect to the theoretical resistance. This improvement is particularly marked in this example when the flocculant dosage is 32 g per kg of dry sludge.

Example 6 Effect of the Botanical Origin Three solutions of different botanical origin are prepared with an equivalent procedure and level of cationicity, in order to obtain a fixed percentage of nitrogen equal to 3%. Waxy corn has a high proportion of amylopectin, while pea is richer in amylose than potato.

These solutions are tested in Jar-Test as a flocculant for the slurry of Example 1, together with the PAM solution, with an R ratio of 0.8, for a total of 18 g / kg of dry sludge. A reference test is performed with polyacrylamide alone at an equivalent dose, ie 3.6 g / kg of dry sludge. The protocol is the same as in Example 1. The results are given in Table 10.

Table 10 Origin Flocs Flocks Turbidity% of Volume of filtrate (ml / s) of beaker sieve of filtrate flocs in starch (UTN) filtrate lOs 30s 300s Without 2 1 70 57 150 190 300 starch Maize 7 7 16 6 190 250 305 waxy Apple 8 5 14 6 260 290 315 soil Pea 5 4 13 10 265 295 325 Flocculation is good, irrespective of the botanical origin of the starch. However, using potato in the process makes slightly better results than using pea or waxy corn. Indeed, with the potato base, the amount of flocs passing through the sieve is less compared to the pea, while the filtration rate is higher than for the waxy corn base.

Claims (17)

REVENDICATIONS1. Process for thickening or dewatering a sludge comprising at least: a) a step of adding flocculants to the sludge to be treated; b) a step of flocculation by stirring the sludge thus added to form a mixture of flocs and an aqueous solution; c) a step of mechanical separation of the flocs and the aqueous solution formed in the previous step; d) a step of recovering the aqueous solution and flocs forming a treated sludge; characterized in that: - the added flocculants consist of at least one cationic starch (A) and at least one cationic polyacrylamide (P); the cationic starch (s) (A) comprise a fixed nitrogen mass percentage of at least 2%; and the ratio of the masses (R) mA / (mA + mp) expressed in dry mass is between 0.6 and 0.99. 2. Method according to claim 1 characterized in that R is between 0.7 and 0.97. 3. Method according to claim 2 characterized in that R is between 0.85 and 0.95. 4. Method according to one of the preceding claims characterized in that the cationic starch (A) has a mass percentage of nitrogen ranging from 2.5 to 5%. 5. Method according to one of the preceding claims characterized in that the cationic starch is obtained from maize starch, wheat, waxy corn or from potato starch, preferably from starch. potato. 6. Method according to one of the preceding claims characterized in that the cationic starch is added via a liquid starch composition comprising at least one cationic starch. 7. Method according to one of the preceding claims characterized in that the cationic starch and the cationic polyacrylamide are added separately in step a). 8. Method according to one of claims 1 to 6 characterized in that the cationic starch and the cationic polyacrylamide are added simultaneously during step a). 9. Process according to claim 8, characterized in that the cationic starch and the cationic polyacrylamide are added simultaneously via a liquid composition comprising at least one cationic starch and at least one polyacrylamide. 10. Method according to one of the preceding claims characterized in that the polyacrylamide has a cationicity level ranging from 500 to 5000 peq / g, preferably from 2000 to 4500 peq / g, by measurement of ionicity with the aid of a flow detector of the Streaming Current Detector (SCD) type, by titration with sodium polyethylenesulphonate. 11. Method according to one of the preceding claims characterized in that the total amount of cationic starch and polyacrylamide dry mass is from 1 to 50 g / kg of sludge to be treated (dry / dry), preferably from 3 to 35 g / kg (dry / dry), preferably from 4 to 20 g / kg (dry / dry). 12. Method according to one of the preceding claims characterized in that the sludge to be treated has been pretreated with coagulants agents, for example with the aid of metal salts. 13. Method according to one of the preceding claims characterized in that the sludge to be treated in step a) is derived from industrial sludge, including sludge from the food industry such as sludge from starch mills, paper mills , chemical industries, or urban effluents. 14. Method according to one of the preceding claims characterized in that at least one mechanical separation step c) is performed by treating the flocculated sludge by passing on a drip table and / or by flotation of the sludge. 15. Method according to one of the preceding claims characterized in that at least one mechanical separation step c) is carried out by treating the flocculated sludge by centrifugation and / or by passing through a filter press. 16. Method according to one of the preceding claims characterized in that the treated sludge obtained in step d) has a dryness of at least 4%, preferably at least 15%, preferably at least 20%. 17. Composition comprising at least one cationic starch (A) and at least one cationic polyacrylamide (P) in which the cationic starch (s) (A) comprises a fixed nitrogen percentage of at least 2% and the mass ratio of which (R) mA / (mA + mp) expressed in dry mass, is between 0.6 and 0.99.